1. Field of the Invention
The present invention relates to a display device. In particular, the present invention relates to a display device for displaying a screen image corresponding to an image signal on an optical waveguide plate by controlling a displacement action of an actuator element in a direction to make contact or separation with respect to the optical waveguide plate in accordance with an attribute of the image signal to be inputted so that leakage light is controlled at a predetermined portion of the optical waveguide plate.
2. Description of the Related Art
Display devices such as cathode ray tubes (CRT), liquid crystal display devices and plasma displays have been known.
Ordinary television receivers and monitor units for computers have been known as the cathode ray tubes. Although the cathode ray tube has a bright screen, it consumes a large amount of electric power. Further, the depth of the entire display device is large in proportion to the size of the screen. Still further, the resolution is deteriorated at the peripheral portion of a displayed image, the image or the graphic is distorted, the memory function is not effected, and it is impossible to make a large display, for the following reason.
That is, the electron beam, which is radiated from the electron gun, is greatly deflected. Therefore, the light emission spot (beam spot) is widened at the portion at which the electron beam arrives at the fluorescent screen of the Braun tube, and the image is displayed obliquely. As a result, the distortion occurs in the displayed image, because there is a certain limit to maintain the large space in the Braun tube in vacuum.
On the other hand, the liquid crystal display device is advantageous in that the entire device can be miniaturized, and the display device consumes a small amount of electric power. However, the liquid crystal display device is inferior in luminance of the screen, and the field angle of the screen is narrow. Further, the arrangement of a driving circuit is extremely complicated, because the gradational expression is performed on the basis of the voltage level.
For example, when digital data lines are used, the driving circuit therefor comprises a latching circuit for holding component RGB data (each 8-bit) for a predetermined period of time, a voltage selector, a multiplexer for making changeover to voltage levels of types corresponding to a number of gradations, and an output circuit for adding output data from the multiplexer to the digital data lines. In this case, when the number of gradations is increased, it is necessary to perform the switching operation at an extremely large number of levels in the multiplexer. Therefore, the circuit construction is complicated.
When analog data lines are used, the driving circuit therefor comprises a shift register for aligning, in the horizontal direction, component RGB data (each 8-bit) to be successively inputted, a latching circuit for holding parallel data from the shift register for a predetermined period of time, a level shifter for adjusting the voltage level, a D/A converter for converting output data from the level shifter into an analog signal, and an output circuit for adding the output signal from the D/A converter to the analog data lines. In this case, a predetermined voltage corresponding to the gradation is obtained by using an operational amplifier in the D/A converter. However, when the range of the gradation is widened, it is necessary to use an operational amplifier which outputs a highly accurate voltage, resulting in such drawbacks that the structure is complicated and the price is expensive as well.
The plasma display has the following advantages. That is, it is possible to realize a small size, because the display section itself occupies a small volume. Further, the plasma display is comfortably viewed, because the display surface is flat. Especially, the alternating current type plasma display also has such an advantage that it is unnecessary to use any refresh memory owing to the memory function of the cell.
As for the plasma display described above, in order to allow the cell to have the memory function, it is necessary to continue the electric discharge by switching the polarity of the applied voltage in an alternating manner. For this purpose, it is necessary that a driving circuit has a first pulse generator for generating the sustain pulse in the X direction, and a second pulse generator for generating the sustain pulse in the Y direction. Therefore, the arrangement of the driving circuit is inevitably complicated.
On the other hand, in order to solve the problems concerning the CRT, the liquid crystal display device, and the plasma display as described above, the present applicant has suggested a novel display device (see, for example, Japanese Laid-Open Patent Publication No. 7-287176). As shown in FIG. 15, this display device includes actuator elements 400 which are arranged for respective picture elements (image pixels). Each of the actuator elements 400 comprises a main actuator element 408 and a substrate 414. The main actuator element 408 includes a piezoelectric/electrostrictive layer 402 and an upper electrode 404 and a lower electrode 406 formed on upper and lower surfaces of the piezoelectric/electrostrictive layer 402 respectively. The substrate 414 includes a vibrating section 410 and a fixed section 412 disposed under the main actuator element 408. The lower electrode 406 of the main actuator element 408 contacts with the vibrating section 410. The main actuator element 408 is supported by the vibrating section 410.
The substrate 414 is composed of ceramics in which the vibrating section 410 and the fixed section 412 are integrated into one unit. A recess 416 is formed in the substrate 414 so that the vibrating section 410 is thin-walled.
A picture element assembly 420 for obtaining a predetermined size of contact area with respect to an optical waveguide plate 418 is connected to the upper electrode 404 of the main actuator element 408. As shown in FIG. 15, the picture element assembly 420 is located near the optical waveguide plate 418 in the ordinary state in which the actuator element 400 stands still. The picture element assembly 420 contacts the optical waveguide plate 418 in the excited state at a distance of not more than the wavelength of the light.
The light 422 is introduced, for example, from a lateral end of the optical waveguide plate 418. In this arrangement, all of the light 422 is totally reflected in the optical waveguide plate 418 without being transmitted through the front and back surfaces thereof by controlling the magnitude of the refractive index of the optical waveguide plate 418. In this state, a voltage signal corresponding to an attribute of an image signal is selectively applied to the actuator element 400 by the aid of the upper electrode 404 and the lower electrode 406. Accordingly, the actuator element 400 stands still in the ordinary state or is displaced in the excited state. That is, the picture element assembly 420 is controlled for its contact and separation with respect to the optical waveguide plate 418. As a result, the scattered light (leakage light) 424 is controlled at a predetermined portion of the optical waveguide plate 418, and a screen image corresponding to the image signal is displayed on the optical waveguide plate 418.
This display device has, for example, the following advantages. That is, (1) it is possible to decrease the electric power consumption, (2) it is possible increase the screen luminance, and (3) it is unnecessary to increase the number of picture elements as compared with the black-and-white screen when a color screen is constructed.